1. Field of the invention
The invention relates to methods and apparatus for forming a mechanically isotropic liquid crystal polymer film. It relates more particularly to producing a film which inherently maintains its flat shape and has a more uniform coefficient of thermal expansion than has been obtainable previously. It also relates to methods and apparatus for forming a film structure comprising two relatively thin outer layers which are controllably oriented in one direction, and one or more relatively thick inner layers controllably oriented in one or more different directions.
2. Description of Related Art
The invention relates in general to the formation of multiaxially oriented films from high-molecular-weight liquid crystalline lyotropic or thermotropic polymers (homopolymers, copolymers, and the like), under processing conditions whereby the films have a controlled molecular orientation. The films of the present invention are preferably prepared from rod-like, extended-chain, aromatic-heterocyclic polymers. These polymers generally fall into two classes, those that are modified in solution form, i.e., lyotropic liquid crystalline polymers, and those that are modified by temperature changes, i.e., thermotropic liquid crystalline polymers. For a shorthand expression covering both types of polymers, the present disclosure will use the term "ordered polymers" or "liquid crystal polymers."
The ordered polymers concerned herein are believed to have a fixed molecular shape, e.g. linear, or the like, due to the nature of the monomeric repeating units comprising the polymeric chain. Linear ordered polymers are also known as "rod-like" polymers. These rod-like polymers can be blended with the more common, typical "coil-like" polymers in which the molecular chain does not have a relatively fixed shape. Some of these blends have processing and functional characteristics similar to liquid crystal polymers, and to that extent, these blends are to be considered as being included in the invention disclosed herein.
Liquid crystal polymer films have desirable qualities in a number of applications, but significant drawbacks related to their mechanical anisotropy. They are useful in particular for forming circuit substrates. Circuits can be formed on such a film by plating and etching, and then a plurality of such circuits can be laminated, to form a circuit board having multiple circuits accommodated within the board. Flexible circuits can also be formed on liquid crystal polymer films,
However, the mechanical properties of liquid crystal polymer films have been inadequate for these applications. They cannot be blown and drawn after extrusion as coil polymers can, since they become too highly oriented in the die. They are too weak in the non-orientation directions to be stretched after extrusion, even while in semi-flowable form. To improve their strength, liquid crystal polymer films are typically extruded between a pair of concentric counter-rotating cylindrical dies to form a tube. This process causes the inner and outer surface layers of the tube to have different respective directions of fibrillar orientation, and this gives the tube biaxial strength and permits blowing and drawing, if desired.
FIGS. 2A-2C are schematic representations of extruder films showing the morphology of the oriented polymer material layers therein. In FIG. 2A, with no transverse or circumferential shear, the film has a uniaxial orientation, with all molecules oriented in the machine direction, that is, longitudinally with respect to the direction of flow through the die. In FIG. 2B the film has a biaxial orientation. The molecules in the top portions of the film are oriented at an angle of +theta with respect to the machine direction while the portions of the film in the lower part of FIG. 2B are oriented at an angle of -theta to the machine direction. FIG. 2C shows a planar isotropic film wherein the polymer rods lie randomly in the film plane, not strongly oriented at any specific angle with respect to the machine direction.
A biaxially oriented tube can be slit and spread apart to form a flattened film structure. However, the present inventors have observed a problem with this process which was not previously understood. The films thus formed will not lie flat. Although such a film can be flattened by pressing under heat, it has been observed that the film regains its tendency to curl as it continues to cool after pressing. Simply described, the two surface layers of the film inherently have different coefficients of thermal expansion (CTE) axially and transversely to the orientation of its molecules. Generally, the transverse CTE is greater. So as the sheet cools, each layer will try to shrink more in its own transverse direction. But since the two layers are both part of the sheet, the sheet as a whole cannot freely shrink in either direction. This stores stresses in the layers and makes the sheet bistable, whereby it is able to hold a curl about either of two different axes and readily adopts one of these two conditions if an active effort is not made to hold it flat.
As best understood, liquid crystal polymer films made of poly-(p-phenylene-benzobisthiozole) (PBZT) or the like have this curling problem because they are fibrillar, i.e., they comprise relatively straight molecules. The molecules orient strongly in the die and the flowing polymer becomes anisotropic, more so than ordinary coil polymers which tend to randomize. A coil polymer tube or sheet can be strengthened biaxially throughout its entire thickness by blowing and drawing after it exits from the die. Sometimes counter-rotating dies are also used to make conventional polymers more isotropic. But the combination of shearing and stretching is much more critical and difficult to optimize with liquid crystal polymer extrudates, since they readily become highly oriented in the die anisotropically. It may not be possible to stretch the polymer substantially in the direction transverse to its fibrillar orientation
We have found that if counter-rotating annular dies are used, to establish a biaxial or multiaxial (specifically, twisted nematic) orientation of the molecules in the flow, then transverse stretching by blowing of the extruded tube is possible and effective.
But, as mentioned above, such a process forms essentially two layers in the film with complementary orientations, i.e., forming equal but opposite angles, for example +/-45.degree., on either side of the machine direction in which the extrusion has taken place. This has led to the drawback of curling in liquid crystal polymer film sheets made from such extruded tubes. The liquid crystal polymer films become less anisotropic due to the application of transverse shear, but they still curl after cooling, because of the non-uniform CTE phenomenon mentioned above. Curl becomes very significant when the film is orthotropic, i.e., having equal properties in orthogonal directions in the plane of the film, as in the desirable balanced biaxial film.
Another problem frequently associated with films produced by the tubular bubble process is seaming. Seams have been formed in some known methods in which film tubes are flattened or "blocked" as they are driven through pinch rolls.
These problems relate at least in part to inherent characteristics of the tubular extrusion process, and in part to the methods of system control and downstream processing, beginning with coagulation or cooling, and perhaps in part also to inadequate dope homogeneity upstream.
No techniques previously known to the art have been able to solve these problems
U.S. Patent Application Ser. No. 206,137 now U.S. Pat. No. 4,963,428, filed Jun 13, 1988; Ser. No. 203,329 filed Jun. 7, 1988 now U.S. Pat. No. 4,939,235; and Ser. No. 098,710 filed Sept. 21, 1987 now U.S. Pat. No. 4,973,442; all commonly assigned herewith, disclose processes wherein biaxially oriented, substantially two-layer, liquid crystal polymer films are formed in counterrotating annular dies by controlling the transverse shear speed, the material flow rate, the blow ratio and the draw ratio, all of which affect the molecular orientation in the final product, to obtain a substantially +/-45.degree. orientation of the two surface layers. See also U.S. Pat. Ser. No. 209,271 filed Jun. 20, 1988 now abandoned.
Nagasawa et al., Japanese Disclosure No. 53-47460, discloses a manufacturing method for a lyotropic liquid crystal polymer film which includes applying transverse shearing forces to the dope. See FIG. 2 and pp. 8-9.
Other prior art of interest includes: PA1 Urasaki, Japanese Disclosure No. 53-86798 PA1 Sugimoto et al , Japanese Disclosure No. 54-44307 PA1 Fujii et al , Japanese Disclosure No. 63-199622 PA1 Fujii et al., Japanese Disclosure No. 63-173620 PA1 Inada et al , Japanese Disclosure No. 52-109578 PA1 Miyamoto et al., Japanese Disclosure No. 63-296920 PA1 Donald, U.S. Pat. No. 3,279,501 PA1 Sharps, Jr., U.S. Pat. No. 3,404,203 PA1 Sharps, Jr., U.S. Pat. No. 4,496,413 PA1 Isayev et al., U.S. Pat. No. 4,728,698 PA1 Helminiak et al., U.S. Pat. No. 4,377,546
These problems in the art are substantially solved by the processes and apparatus disclosed herein, namely bi-annular or tri-annular tubular dies.
Regarding the term "layer," this disclosure will relate at times to a laminated film structure comprising a number of individual intermediate-product films; and at other times to an integral film structure with different planar regions parallel to its main surfaces which are in some respects analogous to individual films, and having different properties in the various planar regions. It is to be understood that the teachings throughout this disclosure are equally applicable to both these forms of liquid crystal polymer film The use of a term such as "layer" should be understood to refer equally to a planar region within an integral film; as well as to an individual intermediate-product film, or a portion thereof, within a laminated structure.
The respective disclosures of all the prior art materials mentioned herein are expressly incorporated by reference.